What's a good "portable" closed back headphone?

And for this, I really do mean portable and not transportable. The closed back's I currently own are the TR-X00 Ebonies and AudioQuest NightOwls and very much enjoy the sound sig on both. Issue is, they are a pain in the butt to carry around even when I have my work/travel bag as their cases just take up a good bit of room. Also, neither isolates very well and the Fostex tends to leak sound like mad.
So far, I have been thinking of the ATH M50x as it's collapsible and and seems sturdy enough to be tossed around in my bag without breaking. I've also looked at the V-Moda M100 for similar reasons. Although they don't collapse, the Sennheiser HD 569/598 Cs look to be very lightweight, sturdy, and overall not very big. I've also seen the Thinksound ON2's here for a good price but I'm a bit apprehensive to get them due to them being more on-ear than over-ear. Their sound sig does seem to be up my alley and they do appear to be rather portable.
For context, I've tried the Status Audio CB-1 in the past and did not like them. I've also tried the Meze 99 Neo's and while I enjoyed the sound, the pads were very uncomfortable and made my ears sweat like mad. The Beyerdynamic COPP's were quite comfy but didn't have the SQ to back it up (my pair had some very bad cup/driver rattle). I also returned some Aeon Flow Closed due to their sound despite them having great build quality, accessories, and comfort (SQ way too lean and cold for my taste).
Price is of no concern as long as they sound good and are truly portable. I tend to prefer cans that have swappable pads (velour vs leather) and cables (length and/or mic'd) but if the headphone is good enough then I guess those two things won't be missed. I also vastly prefer over-ear vs. on-ear.
Any and all suggestions are welcome. Many thanks.

I had original Ath-M50, Beyerdynamic DT770 250 ohm and B&W P5 before, now endup using Sennheiser HD 598 Cs, very satisfied with the sound, super comfortable pad, I even fall a sleep couple times wearing it, easy blending in crowded communal place, best soundstage close headphones I ever had

Sennheiser HD1 BT over the ear HPs are my go to cans for mobile use (you can run ‘em wired or wireless)...& they fold for easy carry in the accompanying semi-soft shell case; I typically wear ’em around my neck...they are effortless to use & lighweight...& the SQ signature is classic Sennheiser goodness.
Note: You can also check out the newly released Sennheiser Momentum wireless ear buds (available for pre order); I ordered a pair for mobile options while I’m out (exercising primarily or in office).
One other real world mentionable:
***I have the TR X00 Fostex black edition, Kenningtons Master Pro...in my closed can stable; Aeon Flow Open & Sennheiser HD 6XX in my open can stable. The Sennheiser Momentum (HD1) over the ear cans easily see the most DAILY use...as I’m constantly prone to listening to music via my phone (over my 2 channel stereo or PC HP desk system).***

As someone who flies ~110K miles/yr only two things matter: comfort and noise cancellation/isolation. I've traveled with both ATs the M40 and M50, neither does a good job at keeping noise out and boy do they get super hot (physically, not musically). Bose is heavily scoffed at and looked down upon, but honestly at 30K feet with turbines hitting 50-60 dBs noise cancellation becomes non-negotiable. While I wouldn't touch Bose with a 10-foot pole on land (and specially at home), you can't beat them for travel: long battery, excellent NC/NI and bearable SQ.

Your taste in audio seems to be very similar to mine. Hopefully, you will find my insights helpful.
What I have that's travel worthy:
Soundmagic E10BT - Bluetooth IEM's, 12hr battery, cut off at the freq. ends but midrange is magic & has that PRAT factor. My go-to when traveling light.
Blue Ever Blue 1200EX - IEM's, took forever to break in (60+hr), Very good bass & overall detail but a little hot on top. V-shaped response. These are probably my least favorite but slap a Bluewave GET on to them & it smooths out that hot treble turning them into magic! (The GET does the same for the 58X as well!)
Sennheiser HD 25 - On ear, original model. Wonderful midrange & soundstage but cut off at both frequency extremes with no a lot of bass detail. Bullet-proof! Power hungry.
Sennheiser HD1 - On ear with a very similar sound sig to the HD 25's but with better extension on both frequency ends. (On loan to niece)
JBL E55BT Quincy Edition - Bluetooth, Over ear. Decent passive noise reduction. A good solid pair of fun, every day headphones that do nothing terribly wrong. They are warm sounding with a tube-like quality. They do lack a bit in the area of detail retrieval though. Very efficient w/ high volume.
Sennheiser PXC 550 - Bluetooth, Over ear, Active NC. My favorite with excellent sound & high PRAT factor. Not a throw-around pair of headphones. They do fold & travel well in case though. Software app is pretty good.
B&O H7 - Bluetooth, Over ear (barely). Wonderful sounding headphones with a few minor quirks. Detail in the upper range is excellent but the midrange can sound a little recessed on some material. They're the best looking & best built of the bunch. Definitely not for throwing around! Noise isolation is lacking as is the software app.
I am seriously considering the German Maestro 8.35D as a good, bullet-proof, throw-around pair of travel headphones.

Honestly for portability and the tons of reviews that I have read. When the drop returns you should roll with the thinksound on2. It had amazing reviews for multiple critics. Unfortunately I didn't get a chance to get a pair but I wish I had grabbed it in the last drop. I ended up choosing the hifiman 4xx. Which this is my first planar magnetic which I do like. But I wish I would have kept the drop for the thinksound on2.

The ATH M40x is a nice lightweight can for not too much money (and I prefer it's sound to the M50x).
Step up to the MSR7 for a bit more cash and "refinement."
Personally though I love my Sony WH1000XM2.

Looks like you're pretty set against the on ear option, but the HD25 (at least in the 25-1 II variant) is fantastic, light, durable, and while not a huge concern for many, rebuildable.
Swappable pads, removable cable (with the same plugs Senn uses on the HD580/6xx and whatever else it's popped up on), and they are essentially designed for broadcasting, so isolation is pretty solid for what you're getting in terms of size and clamp.
Better options out there I'm sure, but my HD25-1 II gets a solid bit of my ear time, portable or not.

My current favorite closed-back(s) are the 1more Triple Over-Ears and the Takstar Pro 82
The 1more's are raved and are even on Tyll's wall if that's worth anything to you. I'm also a bit biased towards 1more and have been a fan of their works since the Xiaomi Piston 2, so there's that as well. Only con at glance is the pads - although they are technically swappable, the design of the pads themselves are very much integrated in the tuning of the headphone (hence why some air pockets exists around the cup). Fantastic, but just a bit too power hungry, but I doubt you wouldn't bring a portable dac/amp when you're this invested already. Comfort could be better (on-ear, lot of headband pressure). Sound is pretty damn good. Detachable LR 2.5mm cables and collapsible. Considering it's 2 misses on your list, I'd move on to the Takstars:
The Pro 82 I love for being so light, so good (bass port gimmicks either off or middle depending on your genre/mood), and very comfortable. A glance at the design you'll see it's a rip of the Sony MDR1A, which one can assume that they were probably manufactured/sourced by Takstar to begin with. Best general review I can give them is that they are a half-priced, tamer MDR1A. This headphone has a cult following on Head-Fi, where one poster in particular makes comparisons to the Fostex TH-900 due to the Takstar's strong scaling. Detachable 2.5mm. Pads aren't user-changable but still can be done.
I know price isn't a factor but in terms of closed back performance, these haven't disappointed me. FWIW my brother tossed me back his ATH-M50 after receiving a Pro 82 as a gift.

Focal Listen. $149 during Black Friday. Better than the M50x in every way except you cannot rotate the earcups. But more comfortable, better sound signature (less harsh, more bass, without going crazy), great imaging, very efficient. Love em.

True, I'm lucky that my ears fit, just barely. I'm adjusting the headband to the second furthest point, too. I have heard from someone else who claims to have a large head that the Listen weren't for them.

The ATH-M50x are the best bang-for the buck. They fit very comfortably on the neck, the clasp is good, very comfortable. Detachable cable. Very durable. Elegant design. The sound is stellar and overall neutral with a little extra bass and treble when compared to the rest of the series. They are very portable, but a bit on the heavy side, the M40x is a less heavy (and cheaper) design, some may even prefer the M40's sound signature.
Now for cons, I have found that if you don't position them well, the top headband will hurt your head a little, over time. I haven't tried many headphones, so I am not a very good source, but I do recommend looking at reviews.

M-100s for me. Not necessarily for their stellar sound, but that they are fun, comfortable, isolate well, pack down to nothing in their case and are built like tanks. I go back to these every time while on the go. Sound is as you’ll read everywhere. V-shape FR but detailed enough to make them “fun” vs “audiophilic.”

At this point, it's almost close to being a toss-up between the ATH M50x and the V-Moda M-100. Both are on the Wall of Fame on InnerFidelity and seem to be durable and actually portable. Have you heard the M50x by chance or know how these two compare?

I have tried them. Bought some for my dad a few years ago, admittedly so that I could give them a try. :) It‘s tough to remember how they stack up and I’m probably biased since I had the M-100s first. I remember the M-100 more bass centric and dynamic (aka “fun” like Tyll said) while the M50x being a bit more musical and clear. Bottom line for me is that the M-100 folds up to a little ball with a sturdier feel and the ear ups are more rounded, hence comfortable to me. I literally have thrown them around for years and they keep on going.

One that I think often gets overlooked is the Sennheiser HD1 (aka, Momentum 2.0). Good sounding, looking, and truly portable. I had the chance to pick one up at a great price, and kind of got it on a lark. I've been positively surprised how well they've done the traditional Senn sound in a portable package. It's been a while since I've heard the 598cs, but to my memory the HD1 compares favorably. Super easy to drive as well.
I've mentioned elsewhere that it's surprising how often I sit amongst my thousands of dollars of audio equipment listening to them straight from my phone.
Two caveats: first, fit can be hit or miss, so definitely try them on. I found them a bit tight at first and had to do some fit break in, but once dialed in, they are great. Second, the over ear is definitely to be preferred. On the strength of the wired over ear, I snagged the wireless on ear. They are, admittedly, a guilty pleasure to this non bass head, but the fit is less comfy and the tuning is as though they managed to fit a 12 inch sub into an otherwise standard Senn can.

Thanks. I remember looking at the Momentum's forever ago and completely forgot about them once I got into the HD5 and HD6 family. Added to the shortlist. Which model is the over-ear? Is that the non-wireless version? The listings on Amazon are kinda all over the place.

Yes, the naming convention is all over the place. There are wired and wireless versions of both on-ear and over ear, but stores just seem to make stuff up when they describe them. Technically, the wired over ear is plain old HD1. But here's a link to the one I own, for reference: https://en-us.sennheiser.com/hd1-over-ear-headphones-with-mic

I'd recommend checking out the Campfire Audio Cascade... excellent isolation, folds up into a smaller case nicely, and bass, clarity, and dynamics that excel beyond a TH-X00. Sound great from most anything you plug them into as well, not picky at all or hard to drive. They do have a weird, not so good imaging issue with the middle 1/3 width of their soundstage and are horrible at gaming because of it and the slightly more intimate soundstage than a TH-X00. Pads are nice and comfortable for their weird slotted shape though, very deep. Not inexpensive though... If they were less expensive I would probably get them.

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Second one I like is my B&O H7... wireless with great sound quality and good enough isolation. Have it paired with my Shanling M0 DAP and enjoying it via aptX bluetooth right now. An on ear but slightly better in comfort than the 1-More Triple Over Ears to me. Discontinued, the H9i would be their replacement.

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Either one is nice soundwise if you enjoy the TH-X00 series headphones.

I do like it quite a bit. Great for portability, and prefer it to the Audioquest Dragonfly Red when used with my modded Gustard H10 amp. The B&O H7s sound IMPRESSIVELY great from it, and that's just via aptX. Will have the Audeze Mobius coming in soon, which is one of the reasons I picked up the M0 - for LDAC comparison to lower rate codecs. Yeah, I was hoping my TAB S2 would have an upgrade to Oreo, but that's not going to happen from what I've been reading. M0 also works great for car audio via bluetooth... just have to control volume via the M0 though.

No need for Shanling in my car, I use Android Auto from my V20. I use the MDR-1000X (with LDAC capability) but the newer releases with both LDAC and AptXHD are appealing as the V20 (at the moment) only supports AptxHD

I own the TH-X00 with Dekoni Sheep skin pads, awesome. soundwise the Meze 99 Classics maple with first pad version are great, too. If you search for Bluetooth cans go wirh the Audio-Technica DSR9BT. Same level as the TH-X00.
Last but not least I modified my Aeon Flow Open with Carbon layers to a semi closed headphone. Fantastic. Details are imoressive, bass is a bit pronounced. No lean sound and a stage like the open one.

My favorite portable headphone (non-iem/non-bluetooth) is the Audeze Sine. I don't have the world's largest ears so they are mostly over-ear for me but they won't be over-ear for everyone. They are also pretty hard to drive for a portable can (need lots of current). If you want to take top tier fidelity on the road, in a fold flat package they can't be beat IMO. My only qualm is the amping front, even my V20 can't put out the juice the need for low level recordings. (V20 see's them as low impedance aka "easy to drive" so it doesn't output full power, I can trick it with some adapters but they are a pain).

The impact to the frequency response is quite headphone dependent. Planar technologies and single driver iem's often yield little to no effect. It's not ideal in any regards which is why I wish they would just make the mode selectable. The damn thing is essentially an automatic gain switch. I just want to select it manually and not have LG set some arbitrary 50 ohm threshold. My hardest to drive cans are at or below that outside of the T1. V20 actually does quite well with higher impedance cans, it only fails with current hungry low impedance planar cans IMO. One could always add pre-amp boost in their source app but that too is less than ideal and not really addressing my core concern.

Yep, I have read the same thing about planars, but given using aux mode is just as easy an option, each requiring a cable extension, I suppose I would be more inclined to avoid invoking high impedance mode...
But now you have me curious... I actually have a couple of 75 ohm in-line adapters lying around (from before I learned of the aux mode approach) so I guess I will have to try A/B-ing with the planar headphones I have (M1060, Sundara, Aeon Flow Closed), regular cable extenders and 75 ohm adapters, and my V30.